Turboexpander-generator units are commonly used machines for converting power contained in a flow of compressed gas into useful mechanical power available on an output shaft of the turboexpander. The output shaft can be used to mechanically drive a rotating machine, for example an electric generator. Integrated turboexpander-generator units usually comprise a gas tight casing housing a turboexpander with a rotating wheel or a plurality of rotating wheels, provided with blades, mounted on a rotating shaft. The shaft is supported for rotation in the casing and is mechanically connected to the rotor of an electric generator, the stator whereof is stationarily mounted in the casing.
The rotating shaft, whereon the rotating parts of the turboexpander and the electric generator are mounted, is usually supported in the casing by means of a plurality of bearings. In known embodiments oil lubricated bearings are used for this purpose. More recently, active magnetic bearings have been developed for applications in turboexpander-generator units. Active magnetic bearings avoid the need of lubricating oil and therefore overcome drawbacks related to the presence of oil in the gas flow through the turboexpander-generator unit.
However, active magnetic bearings are complex machine components, which furthermore require an electronic control through dedicated electronic control units. A complex wiring connecting the control unit to the magnetic bearings is required, which makes assembling of the turboexpander-generator even more complicated.
The need therefore exists for a more efficient, less expensive turboexpander-generator unit, which is simpler to design, assemble and maintain.